roCaptionRenderer bug in Roku 4

On Roku 4, TTML based captions have stopped showing in my app. The same code works fine on the older Rokus(e.g. 1, 2, 3, stick)

I use the roImageCanvas approach to display captions in my app using mode 1:

m.captions = m.player.getCaptionRenderer()
m.captions.setMode(1)
m.captions.setScreen(m.canvas)
m.captions.setMessagePort(m.port)
m.captions.ShowSubtitle(true)

The content metadata for the video has the SubtitleConfig set up as this:

{
  Stream: {URL: "<hls video url>"},
  SubtitleConfig: {TrackName: "<ttml url>"},
  StreamFormat: "hls"
}

Is this a known issue? I am seeing this with firmware version 7.0, build 8792
The Rokus where this works are having the version 7.0, build 9021. This works fine with version 6.2 as well.